Autumn migration tracks of Helopsaltes grasshopper‐warblers from Northeast Asia support recent taxonomic assignments

Abstract Migration strategies are genetically inherited in most songbirds, and closely related species can exhibit markedly contrasting migration programs. Here, we investigate the autumn migration of one Helopsaltes grasshopper‐warbler from a population near Magadan, North East Russia, based on light‐level geolocation. Although often considered to belong to Middendorff's Grasshopper‐warbler H. ochotensis, recent genetic studies suggest that birds from this population are more closely related to Pallas's Grasshopper‐warbler H. certhiola. We compare the migratory behavior of the Magadan bird with two Pallas's Grasshopper‐warblers tracked from populations in the Kolyma River valley and the Amur region, Russia. We found similar migration patterns in all three tracked individuals, with stopover sites in eastern China and wintering sites in mainland Southeast Asia, within the known range for Pallas's Grasshopper‐warbler. Furthermore, based on morphological data compiled during bird ringing, we were able to confirm the presence of potential “Magadan grasshopper‐warblers” during spring and autumn migration in Thailand. Our scant data provide further evidence that Magadan Helopsaltes, notwithstanding their morphological resemblance to Middendorff's Grasshopper‐warbler, constitute a population of Pallas's Grasshopper‐warbler.


| INTRODUC TI ON
Until recently, the migration strategies of small landbird species of the East Asian flyway were relatively poorly studied in comparison with those of the European-African and the American flyways (McKinnon & Love, 2018). This changed considerably during the last decade following the development of tracking techniques based on light levels (see Yong et al., 2021 for review). Tracking studies of East Asian landbirds showed that while a few species of small-and medium-sized species undertake direct sea crossings (Koike et al., 2016) most generally avoid prolonged marine transits, performing detours around water barriers even when they start migration from the islands of the Pacific Ocean (Ktitorov et al., 2022;Yamaguchi et al., 2021;Yamaura et al., 2017). Two main migration corridors of East Asian landbirds were found: first, a mainland route through continental East Asia for populations spending the nonbreeding season in India or mainland Southeast Asia, and second, an island route, with birds migrating from North-East Asia through the chain of islands in the Pacific to nonbreeding sites on the Philippines or Indonesia (Heim et al., 2020). A few species deviate from these general patterns. For example, a Blue-and-white Flycatcher Cyanoptila cyanomelana was tracked from its breeding grounds in mainland Russian Far East to a wintering site on the islands of the Philippines , and Arctic Warblers Phylloscopus borealis breeding in Alaska and wintering in Philippines and Palau might follow a loop migration pattern, taking a mainland route during autumn migration, but the island route during their spring migration (Adams et al., 2022).
Two closely related species that are believed to differ in their migration routes and nonbreeding locations are found within the  International, 2022;Gill et al., 2022;Kennerley & Pearson, 2010).
However, the individuals of the Magadan population display traits that accord precisely with neither Middendorff's nor Pallas's Grasshopper-warblers. Due to their intermediacy in both plumage characters and body size, they have long been considered a possible hybrid lineage (Kalyakin et al., 1993;Sleptsov et al., 2021). Birds from Magadan are larger, less rufescent, and much less streaked than individuals of the geographically closest subspecies of Pallas's Grasshopper-warbler H. c. rubescens, which occurs further to the northwest in the valley of the Kolyma River, but with which they may come into contact. However, the range of the Magadan birds is disjunct from that of topotypical H. o. subcerthiola from Kamchatka which they more closely resemble (Kennerley & Pearson, 2010).
Genetic studies have placed individuals from the Magadan population in the H. certhiola clade (Alström et al., 2018;Drovetski et al., 2015), contradicting their inclusion within H. ochotensis as presently implied by current major world bird checklists (BirdLife International, 2022;Gill et al., 2022). A subsequent genetic and biometrical evaluation has additionally confirmed that other contentious regional Helopsaltes populations, from the lower Amur River, and from northern Sakhalin, are hybrids between Pallas's and Middendorff's Grasshopper-warblers (Evtuch, 2020).
In this study, we investigate the migration strategy of the "Magadan grasshopper-warblers" and compare their nonbreeding spatiotemporal distribution to what is known of the winter distribution of Pallas's and Middendorff's Grasshopper-warblers. The spatiotemporal migration program is heritable and genetically controlled, so hybrids descending from parents with different migratory orientations might be expected to show "intermediate" or "hybrid" migration patterns (Delmore & Irwin, 2014;Sokolovskis et al., 2023).
In general, differences in migratory directions are important to separate otherwise closely related taxa, as different routes can, for example, imply differences in arrival at the breeding grounds that can support speciation processes (Irwin & Irwin, 2005). This allows us to make the following assumptions for the case of the North-East Asian grasshopper-warblers: If the birds from the Magadan population should take a continental route to wintering sites in mainland Southeast Asia or western Indonesia, this would therefore provide evidence that their genetic migration program is closely related to that of the Pallas's Grasshopper-warbler. A route through the islands of the Pacific toward a wintering ground in the Philippines, on the contrary, would suggest that migration genes of the Magadan population were acquired as a result of gene drift from Middendorff's Grasshopper-warblers.

| Geolocator tracking
We used light-level geolocators to track the migration of grasshopperwarblers (Intigeo W50B11, Migrate Technologies, UK) weighing 0.5 g (<4% of the birds' body weight). In 2020, we tagged nine male grasshopper-warblers of the hybrid population near Magadan We analyzed the light data following Lisovski et al. (2020) using R version 3.4.1 (R Core Team, 2021). Twilights were defined using a threshold of 0.8 with the BAStag package (Wotherspoon et al., 2013). We adjusted the start times of the geolocators' internal clocks by adding 12 h. We used a breeding site calibration (24 June to 9 July and 5 July to 8 August in BW944 and BW955, respectively) to estimate the sun elevation angle for each device during the stationary period (Lisovski et al., 2020). Periods of residency were distinguished using the changeLight function with a quantile of 0.9 and 1 day as the minimum duration of stay in the GeoLight package . Zenith angles (−4.05 and −5.34 for BW944 and BW955, respectively) for these periods were estimated using the function getElevation, and coordinates were calculated using the coord function. Final sites were estimated with the mergeSites function and a distance threshold of 200 km.
We defined the day of departure as the first day after a period of residency, and the day of arrival as the first day of stationarity of any given period. We defined stopover time as the sum of days of periods of residency during migration, whereas travel time was defined as the number of days during migration excluding periods of residency. The total duration of migration was calculated as the number of days between departure from the breeding site and arrival at the wintering site for autumn and vice versa for spring migration. Migration distance was calculated as the sum of the great-circle distances between mean positions (including all sites of residency), whereas the direct distance was calculated as the great-circle distance between breeding and nonbreeding site. We estimated travel speed by dividing the migration distance by the number of days spent migrating (total duration of migration minus stopover time), whereas total speed of migration was estimated by dividing the migration distance by the total duration of migration. from Middendorff's, in which the upper-tail coverts are uniform.

| Ringing data
Distinguishing some Magadan birds from Pallas's is less straightforward, but dorsal streaking is usually much more obscure in the former; they always lack any rump streaking and they are substantially larger. Birds with wing chord longer than 70 mm showing hybrid features in coloration are identifiable as male "Magadan grasshopper-warblers," since the other known hybrid population (from Amur and N. Sakhalin) is smaller in size with a winglength <70 mm (Evtuch, 2020;Sleptsov, 2018, Appendix 1).

| Geolocator tracking
All three tracked grasshopper-warblers were found to migrate southwestward along the mainland corridor, with stopover sites in continental East Asia and nonbreeding sites in Southeast Asia.
The initial part of the migration path of birds from the north of the range ran along the northwestern coast of the Sea of Okhotsk  (Table 1). The migration distance was shortest for the Pallas's Grasshopper-warbler from the Amur region, and longest for the Magadan bird (Table 1). Both Pallas's Grasshopper-warblers migrated more slowly than the bird from the Magadan population (221 and 262 vs. 314 km/day). All individuals arrived at their final nonbreeding sites in September, with the bird from the Amur population having the shortest duration migration. Nonetheless, the Magadan individual, in spite of undergoing the longest migration, reached its wintering grounds much earlier than other two birds (Table 1). and were widely sampled and ringed at this and neighboring sites TA B L E 1 Autumn migration details of a grasshopper-warbler from the Magadan population (BW944) compared with two Pallas's Grasshopper-warblers from the Kolyma River valley (BW955) and the Amur region, Russian Far East (BG101), based on light-level geolocation data. Day of departure-first day after a period of residency, day of arrival-first day of period of residency, stopover time-sum of days of periods of residency during migration, travel time-number of days during migration excluding periods of residency, total duration of migration-number of days between departure at the breeding and arrival and the wintering site for autumn and vice versa for spring migration, migration distance-sum of the great-circle distances between mean positions (including all sites of residency), direct distancegreat-circle distance between breeding and wintering site, travel speed-total duration of migration minus stopover time, total speed of migration-dividing the migration distance by the total duration of migration, latest date-last date with geolocation data. For more details, please refer to Material and Methods.

| DISCUSS ION
We compiled autumn tracking data of three grasshopper-warblers, including one "Magadan grasshopper-warbler." The grasshopperwarbler from the Magadan population showed very similar migratory behavior to the Pallas's Grasshopper-warblers from the Kolyma River valley and the Amur region ( Figure 1). The migration of all observed birds followed the mainland migration corridor, as do many other East Asian landbirds (Heim et al., 2020;Yong et al., 2021). All individuals made stopovers in eastern China and migrated to wintering areas in Southeast Asia (Figure 1).
The Pallas's Grasshopper-warbler from the Kolyma River valley most likely made its first stopover on the Schantar islands in the Sea of Okhotsk (Figure 1), suggesting that grasshopper-warblers breeding north of that sea might cross it instead of detouring around it.
This differs from Arctic Warblers tagged in Alaska, which performed a long westward detour through Chukotka and Yakutia before turning south and flying over mainland East Asia toward their wintering grounds (Adams et al., 2022). The low precision of the location estimates likely stems from the grasshopper-warblers' skulking behavior, which causes substantial shading of the geolocators' light sensor . concentration and coincidence in a short period of occurrence, together suggest that our tracked Magadan bird serves as an example of a general migration pattern that is common to the grasshopperwarblers of the Magadan population. It was expected that the bird from the Amur region might start its autumn migration later, given that its breeding site is situated further south and closer to the nonbreeding grounds. Furthermore, southern breeding populations of Pallas's Grasshopper-warblers are known to undergo postbreeding molt on the breeding sites to a larger extent than their northern conspecifics (Eilts et al., 2021), which could also delay their migration.
The somewhat higher migration speed of the Magadan bird (  (Evtuch, 2020;Hahn et al., 2016;Kennerley & Pearson, 2010;Sleptsov et al., 2021). However, given the very low sample size, further studies are needed to confirm this. The

ACK N OWLED G M ENTS
The initial study design was suggested and initiated by A. V.

DATA AVA I L A B I L I T Y S TAT E M E N T
Geolocation data used in this study are publicly available at MoveBank (www.moveb ank.org) in the study "Grasshopper warbler" (MoveBank ID 2667971916).